In satellite communications, separate transmit and receive signals are generated and received at communication satellites. Typically, a satellite includes an antenna dish for transmitting signals of a first radio frequency to another satellite or a ground receive station. The satellite may also include another reflector or antenna dish of another second similar radio frequency. The satellite also can include a combination of different reflectors and antenna dishes with horns for transmitting and receiving still other frequencies. The satellite transmits and receives along a series of different frequencies that extend in a range F1-F2. These frequencies are far enough apart not to create excessive interference with various transmit and receive communications.
However, in any satellite and similar communication system, the feed lines and antennas are not linear devices. They act essentially like diodes and cause non-linear intermodulation. When the signals are transmitted through the feed lines and antennas, these frequencies form different harmonics and intermodulations producing passive intermodulation, which does not introduce any gain in the system. This passive intermodulation occurs through the modulation components of a complex wave. Thus, waves are produced having frequencies that, among others, are equal to the sums and differences of those components of the original wave.
Testing for intermodulation has been very difficult in the past. Even regular antenna testing typically occurs in an anechoic chamber, where many parameters can be measured, such as the antenna gain. These anechoic chambers are also used for testing for passive intermodulation. However, when testing for passive intermodulation, the chamber has much more stringent design requirements. The anechoic chamber is a building that has few echoes, such as produced by reflections of natural and man-made objects. The chamber surface is covered with electromagnetically absorbing cones, which absorb any passive intermodulation and reflected signals that may be developed by doors, screws or even wires in the floor.
The anechoic chamber is also designed so that the area is free of extraneous signals, such as CB radio signals and other interfering or jamming signals. Naturally, these anechoic chambers are very expensive, even more so for those chambers that are designed for testing for passive intermodulation. Because the slightest object could develop passive intermodulation, it is extremely important that those anechoic chambers designed for testing for passive intermodulation have the highest quality parts, naturally incurring even more expense.
One conventional approach used in testing for passive intermodulation in antennas that could be used in satellite communications has been to place two test antennas generating respective frequencies, F1 and F2, within a high quality anechoic chamber. The two frequencies, F1 and F2, are reflected from a reflector, such as an antenna dish, back into a receiver. A spectrum analyzer then measures the passive intermodulation by means known to those skilled in the art. This measuring could include mixing the various orders of frequencies of the input signal within the spectrum analyzer to obtain the tuned frequency. The anechoic chamber attenuates the reflections, which would interfere with the passive intermodulation measurements.
With satellite communications becoming increasingly more important, while also ground communication stations are increasing their use of antenna dishes, it is important that all antenna devices and their associated components, such as feed systems, be tested for passive intermodulation. Thus, unless alternatives are found for the very stringent design requirements necessary for operating anechoic chambers for testing passive intermodulation in antennas, it is mandatory that large expenditures of personnel time, money and other resources be placed into the design, testing, manufacture and operation of these sophisticated anechoic chambers.